In public safety environment, the resources available are typically optimised so that normal day-to-day operations can be carried out by the personnel on duty. In addition, the organisations need to be prepared to call in a number of standby personnel, which can be activated with only a short delay in cases of unexpected increase in the need for resources, e.g. major accidents or other incidents.
The calling in of standby personnel has typically been a very organisation specific procedure involving lists of backup numbers of fixed lines or commercial cellular networks, but the optimal way would undoubtedly be the use of pagers. In paging systems communication is typically unidirectional, i.e. the pager functions only as a receiver. The pagers are typically also lightweight to be easily carried around anywhere, and the operating time of the device is essentially longer than e.g. a single working shift. These basic requirements of paging are essentially fulfilled by commercial off-the-shelf (COTS) pagers, and some advanced new digital public safety systems already provide an interface, which allows easy integration of such systems to the network.
However, as various two-way mobile speech and text services have become widely available, the competitiveness of the commercial paging systems has diminished, and thus the availability of commercial COTS paging services is constantly decreasing. Furthermore, it is mainly against the agreed public safety security policies to make the rapid activation of operations dependent on the availability and encryption level of commercial communication services. Building up and maintaining a separate paging network merely for public users, on the other hand, induces high additional capital and operational costs that the authorities are not willing to take. In this respect the preferred option would be to implement also the paging service over the secure and resilient network with which the operational public safety communication is managed altogether.
The European Telecommunications Standards Institute (ETSI) has defined a standard for mobile public safety communications, Terrestrial Trunked Radio (TETRA). As a modern digital cellular radio system, TETRA offers two way text messaging service, which can be, and to some extent already is, used for paging as well. However, there is a practical problem that has so far prevented extensive use of TETRA Short Data Service (SDS) as a bearer for paging service.
Network planning is an iterative process during which the coverage is optimised to fulfil the qualitative requirements for the service and at the same time to conform to the pre-set financial limits for investment and operation. The TETRA air interface provides efficient means (direct mode operation, repeaters, gateway repeaters, mobile base stations etc.) to facilitate operative communication even in places of weaker coverage (buildings, tunnels etc.) This leads to the practice that in order to control the costs, the network planning of public safety networks is driven by good quality, continuous outdoor coverage, not that much by the availability of exhaustive indoor coverage. Though operational communication is thus excellently facilitated, for paging service this practice, however, constitutes a drawback.
People off shift spend a lot of their time indoors at home, and it is thus probable that the delivery of the paging message can be considerably delayed because of the low indoor coverage. Even though the requirement on the acceptable period for getting the message through is much more lenient in paging than in operative communication, it is clear that such delays in paging due to inadequate indoor coverage are not acceptable in public safety environment.
Another practical disadvantage in paging over the TETRA SDS is the load incurred on the main control channel by paging. Especially in case of a major incident, the control channel is already highly loaded with signalling related to critical operative communication, and thus major additional load due to paging is not acceptable.